Grating structure for directing non-polarized light

ABSTRACT

A grating structure having a fused silica base includes alternating ridges and grooves. The ridges and grooves form a fused silica to air interface. The ridges and grooves are configured such that the grating has a ratio between the effective refractive index difference between s-polarization and p-polarization of about 1/3. As such, for non-polarized light with an incident angle θ in  of between 40° and 90° and a wavelength λ=350-1600 nm, the grating directs both s-polarization and p-polarization components of the incident light to the −1 st  order diffraction mode.

BACKGROUND

1. Field of the Invention

The present invention generally relates to a grating structure.

2. Description of Related Art

Various grating structures have been introduced in industry. Gratings typically have rows of grating lines that diffract light. The diffracted light is generally distributed into a diffraction pattern forming a number of modes. One type of diffraction grating is a transmission grating. Typically, transmission gratings comprise grooves etched into a transparent material. As the elements of light in the incident spectrum strike the grooves at a certain angle, they are diffracted and, therefore, separated to various degrees. In many optical applications, light sources generate diffuse light with randomized polarizations. In these applications, typical gratings waste much of the light and, therefore, are not efficient in many beam conditioning applications.

In view of the above, it is apparent that there exists a need for an improved grating structure.

SUMMARY

In satisfying the above need, as well as overcoming the enumerated drawbacks and other limitations of the related art, the present invention provides an improved grating structure.

In one configuration, the grating structure has a fused silica base. The fused silica base includes alternating ridges and grooves that may be etched into the base. The ridges and grooves form a fused silica to air interface. The ridges and grooves are configured such that the grating has a ratio of the effective refractive index difference between s-polarization and p-polarization of about 1/3. As such, for non-polarized light with an incident angle θ_(in) of between 40° and 90° and a wavelength λ=350-1600 nm the grating directs both s-polarization and p-polarization components of incident light to the −1^(st) order diffraction mode.

Further objects, features and advantages of this invention will become readily apparent to persons skilled in the art after a review of the following description, with reference to the drawings and claims that are appended to and form a part of this specification.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side view of a transmission grating;

FIG. 2 is a perspective view of a transmission grating;

FIG. 3 is a flow chart illustrating a method for producing a grating;

FIGS. 4 a and 4 b are a graphs illustrating the relationship of the effective refractive index difference between p-polarized light and s-polarized light relative to the fill factor of the grating;

FIG. 5 a is a graph illustrating the relationship of the effective refractive index difference relative to angle of incidence;

FIG. 5 b is a graph illustrating the relationship of the height relative to angle of incidence;

FIG. 5 c is a graph illustrating the relationship of the fill factor relative to angle of incidence;

FIG. 5 d is a graph illustrating the relationship of the aspect ratio relative to angle of incidence;

FIGS. 6 a and 6 b are a graphs illustrating the relationship of the diffraction efficiency relative to the height of the grating;

FIG. 7 is a graph illustrating the relationship of the diffraction efficiency relative to the angle of incidence;

FIG. 8 is a sideview of a tail light assembly including a transmission grating; and

FIGS. 9 a and 9 b are graphs illustrating the relationship of the diffraction efficiency relative to the change in incident angle and normalized wavelength.

DETAILED DESCRIPTION

Referring to FIG. 1, a system 10 including a transmission grating 11 is provided. The transmission grating 11 may be a fused silica transmission grating with a silica to air interface. As such, air surrounds a silica structure 12 and is denoted by reference number 14. The silica structure 12 includes a base 16 of solid fused silica. Fused silica is very transparent and transmits a very broad bandwidth of light. Further, fused silica offers a very stable material that can be used over a wide range of temperature conditions. In addition, fused silica gratings may be easily etched to provide the grating properties required for many applications. Fused silica has an index of refraction of about 1.45 in contrast to air with an index of refraction of about 1. The symbol n_(α) is used to denote the refraction index of air and n_(β) is used to denote the refraction index for fused silica.

Protrusions 18 extend from and are integral with the base 16. Being integral with the base 16 the protrusions 18 are also formed of fused silica. The protrusions 18 form grooves 20 located between each protrusion 18. The grooves 20 may be filled with air 14, thereby providing an air fused silica interface across the grating layer 22. The grating layer 22 diffracts light directed towards the transmission grating 11 from a light source into various diffraction modes. Each of the protrusions 18 may form a ridge 38 that extends to provide a uniform line structure, as denoted by lines 40 in FIG. 2. The protrusions 18 may have a top surface 42 and side surfaces 44. The side surfaces 44 may have various profiles or may be substantially straight.

Referring again to FIG. 1, incident light may be provided to the transmission grating 11, as denoted by arrow 30. The incident light 30 has an incident angle θ_(in) relative to the principle axis 31 of the grating projections 18. In addition, the incident light 30 may comprise various light polarizations. For example, the incident light may comprise components that are s-polarized 30A and components that are p-polarized 30B. S-polarization denotes when the electrical field is perpendicular to the plane of light propagation. P-polarization denotes where the electrical field is parallel to the plane of light propagation. When the incident light 30 interacts with the grating layer 22, the incident light 30 will form reflective components denoted by R and transmissive components denoted by T.

The reflective components may form a diffraction pattern comprised of a plurality of modes. For example, the 0 order mode of the reflective component R_(n=0) is denoted by arrow 32. Similarly, the −1^(st) order mode of the reflective component R_(n=−1) is denoted by arrow 34. In addition, the transmission grating 11 is mounted in the Littrow mounting condition. Littrow mounting is the condition that produces the same angle of diffraction for both the −1^(st) and 0^(th) order modes, but in opposite directions. The angle for the −1^(st) order mode is θ_(r,−1), while the angle for the 0^(th) order mode is θ_(r,0).

The transmissive components may also form a diffraction pattern comprised of a plurality of modes. For example, the 0 order mode of the transmissive component T_(n=0) is denoted by arrow 36. Similarly, the −1^(st) order mode of the transmissive component T_(n=−1) is denoted by line 38. Again, the Littrow mounting produces the same angle of diffraction for both the −1^(st) and 0^(th) order modes, but in opposite directions. The angle for the −1^(st) order mode is θ_(t,−1), while the angle for the 0^(th) order mode is θ_(t,0).

The resulting characteristics of the reflective and transmissive components are a factor of the refractive index (n) of the material, the period (p) of the grating, the fill factor (r) of the grating, and the height (h) of the grating. The period of the grating is the distance from the start of one groove to the start of the next groove. The period of the transmission grating 11 is denoted by reference numeral 24. The fill factor (r) can be defined as the ratio of the protrusion width or groove width to the period of the grating, which is denoted by reference numeral 26. The height (h) of the grating is the distance from the top of the protrusion 18 to the bottom of the groove 20, which is denoted by reference numeral 28 in FIG. 1. As one would readily understand, the grooves 20 and protrusions 18 may not form exact right angles and various profiles may be used along the edge 42 of the protrusions 18. As such, the calculation for the fill factor (r) or grating height (h) may be slightly modified depending on the shape of the projections 18 and grooves 20. As such, these values may be determined based on the center of gravity of the projections 18 and grooves 20.

A Littrow mounting condition of the transmission grating 11 having an interface of air/fused silica may be analyzed by modal analysis, and can be derived to provide simultaneously a −1st order diffraction for both p-polarization and s-polarization. The analysis points out that the effective refractive index difference of two propagation modes in the grating has a ratio of 1/3 with an incident angle above 400 to select the minimum aspect ratio of the groove height to the ridge width or groove height to groove width. The grating structure fulfilling this condition exhibits a transmittance of more than 90% and an aspect ratio from 6.6 to 16.8 free space wavelengths for an incident angle from 30° to 50°. A 90° coupler is presented as one application for incoherent light.

With regard to analysis methods, rigorous coupled-wave analysis has an advantage of accommodating various groove shapes. Several shapes of grooves such as semi-circle, rectangular, triangular, and curved surfaces can be used. Coupled-wave analysis is typically used for designing gratings, but due to various assumptions this method would not identify the described parameters. Coupled-wave analysis is a numerical analysis and does anticipate propagation mode and evanescent mode integration. On the other hand, a modal analysis can provide a physical insight of diffraction phenomena, although it has less flexibility to adapt for various groove shapes.

When gratings are used for unpolarized light such as light emitting diodes (LEDs), both p-polarization and s-polarization should be simultaneously taken into account in the design. Particularly, the use of −1st order diffraction extends the design degrees of freedom for optical devices, components, and assembled systems due to large bending of light. P-polarization and s-polarized −1st order diffraction can be achieved with incident angle from 30° to 45° by computer optimization using coupled-wave analysis. However, it was thought that a grating needed a larger height and ridge width to accommodate increasing incident angles.

However according to the method described herein, a Littrow mounting condition of rectangular grating at the interface of air/fused silica may be derived through a modal analysis to provide simultaneously a −1st order diffraction for both p-polarization and s-polarization. The analysis can identify that the ratio of the effective refractive index difference of two propagation modes in the grating for p-polarization and s-polarization depends on incident angle, resulting in different ratios to select the minimum aspect ratio of the grating height to the ridge width of fused silica or the groove width of air.

In the modal analysis, effective index, n_(eff), of excited modes in the grating satisfies the eigen value equation.

$\begin{matrix} {{{{{\cos \left( {\beta \; {rp}} \right)}{\cos \left( {{\alpha \left( {1 - r} \right)}p} \right)}} - {\frac{1}{2}\left( {\frac{\alpha \; t_{\alpha}}{\beta \; t_{\beta}} + \frac{\beta \; t_{\beta}}{\alpha \; t_{\alpha}}} \right){\sin \left( {\beta \; {rp}} \right)}{\sin \left( {{\alpha \left( {1 - r} \right)}p} \right)}}} = {\cos \left( {{kn}_{\alpha}p\; {\sin \left( \theta_{in} \right)}} \right)}}\mspace{79mu} {where}} & (1) \\ {\mspace{79mu} {\alpha = \sqrt{\left( {kn}_{\alpha} \right)^{2} - n_{eff}^{2}}}} & \left( {2a} \right) \\ {\mspace{79mu} {\beta = \sqrt{\left( {kn}_{\beta} \right)^{2} - n_{eff}^{2}}}} & \left( {2b} \right) \\ {\mspace{79mu} {t_{\alpha} = \left\{ \begin{matrix} ɛ_{\alpha} & \left( {p - {{pol}.}} \right) \\ 1 & \left( {s - {{pol}.}} \right) \end{matrix} \right.}} & \left( {3a} \right) \\ {\mspace{79mu} {t_{\beta} = \left\{ \begin{matrix} ɛ_{\beta} & \left( {p - {{pol}.}} \right) \\ 1 & \left( {s - {{pol}.}} \right) \end{matrix} \right.}} & \left( {3b} \right) \end{matrix}$

α, β, and t are parameters in equation (1), and defined by equations (2a)-(3b), k is the wave number, and ε_(α) and ε_(β) are permittivity of air and fused silica. In Littrow mounting, the right part of equation (1) equals minus unity. Excited modes are numbered from the largest value of the square of the effective refractive index, n_(eef) ². Within an incident angle θ_(in) from 30° to 90°, the lowest two modes, m=0 and 1, are propagation modes with positive n_(eef) ², while other modes, m≧2, are evanescent modes with negative n_(eef) ². Here, m=0 and 1 are considered with an incident angle θ_(in) from 30° to 85°. Also, the grating may provide the highest diffraction efficiency with the −1st order when the grating height, h, is set so as to have a phase difference of 180° between the lowest two modes, satisfying equations (4a), (4b).

$\begin{matrix} {n_{d} = \frac{\lambda}{2h}} & \left( {4a} \right) \end{matrix}$

where effective refractive index difference, n_(d), is written with

n _(d) =n ₀₌₀ −n _(m=1)  (4b)

where λ is free space wavelength, and n_(m=0) and n_(m=1) are effective refractive index of modes m=0 and 1.

A method for producing a grating according to one embodiment is provided in process 300. In block 310, the wavelength range of light is defined. In block 312, the incident angle of the light is defined. In block 314, the period of the grating is defined based on a Littrow mounting condition. In block 316, the relationship between the effective refractive index difference and the fill factor is determined. effective refractive index difference can be graphed with respect to the fill factor for both s-polarization and p-polarization, as shown in the graph in FIGS. 4 a and 4 b. In block 318, fill factor values are identified, for the above determined parameters, that have approximately a 1/3 ratio between the difference of the effective refractive index for s-polarization and p-polarization. In block 320, a height of the grating is determined for each instance having a 1/3 ratio between the difference of the effective refractive index for s-polarization and p-polarization. In block 322, the height and fill factor are used to determine the aspect ratio of the grating for each instance having a 1/3 ratio between the difference of the effective refractive index for s-polarization and p-polarization. In block 324, the grating may be fabricated, for example by etching, based on the parameters determined in the above noted steps. In one embodiment, the fill factor with the smallest height is selected. In another embodiment, the fill factor with the lowest aspect ratio is selected.

The methodology of designing a p-polarized and s-polarized −1st order diffraction grating, may be accomplished by extending equations (1)-(4). Since the relationship between p-polarizations and s-polarizations is considered, a more general expression is introduced. When grating height, h_(p), for p-polarization or h_(s) for s-polarization satisfies (5), each polarization enhances diffraction efficiency of the −1st order.

$\begin{matrix} {{\left( {n_{d,p},n_{d,s}} \right) = \left( {\frac{\left( {{2i} - 1} \right)\lambda}{2h_{p}},\frac{\left( {{2j} - 1} \right)\lambda}{2h_{s}}} \right)}\left( {{i = 1},2,\ldots \mspace{14mu},{j = 1},2,\ldots} \right)} & (5) \end{matrix}$

The −1st order diffraction of p-polarization and s-polarizations is simultaneously enhanced when required each height has the same physical height, h, as is given by

h=h _(p) =h _(s)  (6)

The p-polarization and s-polarized −1st order condition is written with the ratio of effective refractive index differences, n_(d,p), for p-polarization to n_(d,s) for s-polarization.

$\begin{matrix} {\frac{n_{d,p}}{n_{d,s}} = \frac{\left( {{2i} - 1} \right)}{\left( {{2j} - 1} \right)}} & (7) \end{matrix}$

Then, the grating height, h, is determined by equations (5)-(7).

One element of this design methodology is that the expression of the ratio n_(d,p)/n_(d,s) allows appropriate grating parameters to be found easily according to the variation of fill factor for p-polarizations and s-polarizations. In this design methodology, infinite numbered combinations of (i,j) fulfill equation (7). In practice, an appropriate dimension of the grating may be effectively selected in view of fabrication constraints.

Now referring to FIGS. 4 a and 4 b, the effective index difference is graphed with respect to the variation of the fill factor. The fill factor is the ratio (r) of the grating ridge width of fused silica to the grating period (p). More specifically, line 412 of FIG. 4 a denotes the relationship for s-polarization at θ_(in)=30° and line 414 of FIG. 4 a denotes the relationship for p-polarization at θ_(in)=30°. Meanwhile, line 422 of FIG. 4 b denotes the relationship for s-polarization at θ_(in)=50° and line 424 of FIG. 4 b denotes the relationship for p-polarization at θ_(in)=50°.

All points on the curves of p-polarization or s-polarization provide the maximum diffraction efficiency of the −1st order when the grating height is set to fulfill equations (5). From the view point of fabrication, the minimum aspect ratio of the grating height to the edge width of fused silica or air may be desired. Although infinite numbered combinations (i,j) may satisfy equation (7), the two combinations, (i,j)=(1,1) and (1,3) are considered here. Those correspond to n_(d,p)/n_(d,s)=1 and 1/3.

It can be seen in FIG. 4 a that n_(d,p)/n_(d,s) has a value of 1 at r=0.64 and 1/3 at r=0.21. On the other hand, n_(d,p/)n_(d,s) has 1 at r=0.93 and 1/3 at r=0.04 and 0.54 as shown in FIG. 4 b. Since there are the two values of r for n_(d,p)/n_(d,s)=1/3, the ratios are respectively numbered as 1/3a and 1/3b. However, in reviewing FIGS. 4 a and 4 b not only n_(d,p)/n_(d,s)=1 but also n_(d,p)/n_(d,s)=1/3 should be considered to obtain the lower grating height in both p-polarizations and s-polarizations case, depending on an incident angle in Littrow mounting.

Effective refractive index differences fulfilling n_(d,p)/n_(d,s)=1 and 3 were calculated in θ_(in) from 30° to 85° and plotted in FIG. 5 a. More specifically, line 512 is the effective index difference relative to the incident angle θ_(in) for p-polarization in a first instance (a) where the ratio of the effective refractive index difference is about 1/3. Line 514 is the effective index difference relative to the incident angle θ_(in) for s-polarization in the first instance (a) where the ratio of the effective refractive index difference is about 1/3. Similarly, line 516 is the effective index difference relative to the incident angle θ_(in) for p-polarization and line 518 is the effective index difference relative to the incident angle θ_(in) for s-polarization in a second instance (b) where the ratio of the effective refractive index difference is about 1/3. Finally, line 520 is the effective index difference relative to the incident angle θ_(in) for both s and p-polarizations for the instance where the ratio of the effective refractive index difference is 1.

The angular step width was basically 5° and further more angles were added at discontinuity points. It can be seen by line 520 that n_(d,p) and n_(d,s) exist within θ_(in) from 30° to 58° for the ratio of unity. On the other hand, in case of n_(d,p)/n_(d,s)=1/3, n_(d,p) and n_(d,s) have a first set numbered by 1/3a, in θ_(in) from 30° to 48.5°, and two sets by 1/3a and b, with the further increase of θ_(in).

The grating height was calculated from FIG. 5 a, using equation (5), and is presented in FIG. 5 b. As such, line 532 is the height relative to the incident angle θ_(in), in the first instance (a) where the ratio of the effective refractive index difference is about 1/3. Line 534 is the height relative to the incident angle θ_(in) in the second instance (b) where the ratio of the effective refractive index difference is about 1/3. Finally, line 536 is the height relative to the incident angle θ_(in) where the ratio of the effective refractive index difference is 1.

As illustrated, the grating height increases sharply for n_(d,p)/n_(d,s)=1 when increasing θ_(in) beyond 30°. Alternatively, n_(d,p)/n_(d,s)=1/3 provides a lower height than n_(d,p)/n_(d,s)=1 when θ_(in) is larger than 43° with the further increase of θ_(in).

Accordingly, FIG. 5 c illustrates the corresponding fill factor for each scenario. Line 552 denotes the value of the fill factor for each angle θ_(in) in the first instance (a) where the ratio of the effective refractive index difference is about 1/3. Line 554 denotes the value of the fill factor b for each angle θ_(in) in the second instance (b) where the ratio of the effective refractive index difference is about 1/3. Line 556 denotes the value of the fill factor for n_(d,p)/n_(d,s)=1 at each angle θ_(in). The fill factors for n_(d,p)/n_(d,s)=1, 1/3 a, and 1/3 b all increase with the increase of θ_(in).

In view of fabrication, the aspect ratio of the grating height to the edge width of fused silica or groove is taken to be smaller of the fused silica or groove width. The aspect ratio, AP_(opt), is given by

$\begin{matrix} {{AS}_{opt} - {\min\limits_{l}\left( {\max \left( {\frac{h_{l}}{r_{l}\lambda},\frac{h_{l}}{\left( {1 - r_{l}} \right)\lambda}} \right)} \right)}} & (8) \end{matrix}$

where h_(l) and r_(l) are the grating height and fill factor that fulfill equations (5)-(7). When the fill factor is less than 0.5, the edge width of fused silica is used, and when the fill factor is greater than 0.5 the groove width is selected. In FIGS. 5 a-5 c, the grating parameters of the three cases, n_(d,p)/n_(d,s)=1, 1/3 a, and 1/3 b, are presented. Similarly, FIG. 5 d illustrates the aspect ratio AP_(opt) with respect to the variation of θ_(in), as denoted by line 560. The condition n_(d,p)/n_(d,s)=1 provides the lowest aspect ratio of grating with the groove width of air less than 0.5 from 30° to 37.5° that are represent by n_(d,p)/n_(d,s)=1 and Air at the top of FIG. 5 d. The condition n_(d,p)/n_(d,s)=1/3 provides the lowest aspect ratio with the further increase of θ_(in), where the fill factor exceeds 0.5 at 47.5°. Then, n_(d,p)/n_(d,s) switches from 1/3, a, to 1/3, b at 60.2°. One very interesting and unexpected feature is that n_(d,p)/n_(d,s)=1/3 provides the lowest aspect ratio when θ_(in) is larger than 37.5°, not n_(d,p)/n_(d,s)=1.

FIGS. 6 a and 6 b illustrate the diffraction efficiency with respect to a variation in height. In FIG. 6 a, the incident angle θ_(in)=30°, while the period p=λ, the fill factor r=0.64, and the height h=2.33λ. Line 612 is the s-polarization diffraction efficiency to the −1^(st) order, while line 614 is the s-polarization diffraction efficiency to the 0^(th) order. Line 616 is the p-polarization diffraction efficiency to the −1^(st) order, while line 618 is the p-polarization diffraction efficiency to the 0^(th) order.

In FIG. 6 b, the incident angle is θ_(in)=50°, while the period p=0.653λ, the fill factor r=0.54, and the height h=4.44λ. Line 622 is the s-polarization diffraction efficiency to the −1^(st) order, while line 624 is the s-polarization diffraction efficiency to the 0^(th) order. Line 626 is the p-polarization diffraction efficiency to the −1^(st) order, while line 628 is the p-polarization diffraction efficiency to the 0^(th) order. The grating parameters are consistent with FIG. 5 d.

For the instance where θ_(in)=30° in FIG. 6 a, the p-polarization and s-polarization have the same period with variation of height due to n_(d,p)/n_(d,s)32 1. Diffraction efficiency of −1st order is 94.9% for p-polarization and 96.2% for s-polarization at h=2.33×. On the other hand, the period with variation of height for s-polarization is one third of that for p-polarization with n_(d,p)/n_(d,s)=1/3, when θ_(in)=50° in FIG. 6 b. Diffraction efficiency is 90% for p-polarization and 95.8% for s-polarization at h=4.44×.

Consistent with this method other specific implementations may be particularly useful. In one embodiment, the incident angle θ_(in) is about 40° and the alternating ridges and grooves have a grating period p=0.75λ-0.81λ, a fill factor r=0.32-0.42, a grating height h=4.1λ-4.7λ. In another embodiment, the incident angle θ_(in) is about 50° and the alternating ridges and grooves have a grating period p=0.62λ-0.68λ, a fill factor r=0.49-0.59, a grating height h=4.1λ-4.7λ. In another embodiment, the incident angle θ_(in) is about 60° and the alternating ridges and grooves have a grating period p=0.55λ-0.61λ, a fill factor r=0.63-0.73, a grating height h=5.1λ-5.7λ. In yet another embodiment, the incident angle θ_(in) is about 70° and the alternating ridges and grooves have a grating period p=0.5λ-0.56λ, a fill factor r=0.34-0.44, a grating height h=2.8λ-3.4λ.

FIG. 7 illustrates the best diffraction efficiency to the −1^(st) order for each of conditions discussed with respect to the variation of incident angle. Line 712 denotes the diffraction efficiency for s-polarization and line 714 denotes the diffraction efficiency for p-polarization. The grating parameters are consistent with FIG. 5 d. Further, the angular step width was calculated at 5° steps and further more angles were added at discontinuity points. Diffraction efficiency is higher than 90% for p-polarization and 92.5% for s-polarization from 30° to 50°, and 79.2% and 87.9% with a further increase up to 65°.

One application based on the design methodology is presented in FIG. 8. A light assembly 810, for example as a tail or brake light for a vehicle, comprises a light source 812 and a grating 814. The light source 812 may be comprised of unpolarized lights such as light emitting diodes. Light generated from the light source 812 is denoted by arrow 820. Further, the light 820 includes an s-polarization component and a p-polarization component. The grating 814 may have all of the features described above with respect to the grating 11 and resulting from the processes herein described. As such, the light may be directed at the grating with an incident angle of between 40° and 70° degrees, for example at approximately 55.4°. According to one embodiment, the grating may couple the light at a 90° angle to direct the light out from vehicle, for example through a lens 816.

Light that travels through two different media, such as air and silica, has a bend angle of |θ_(in)−θ_(r,0)|, and the angle is generally larger than 90° when the 0^(th) order diffraction is used. On the other hand, the −1st order diffraction has the light bend angle of θ_(in)+θ_(r,−1), and can provide a light bend angle less than 90°. Thus, a 90° coupler could be built to direct light without additional materials rather than using a traditional 45° inclined mirror. Unpolarized light is directed upon the grating with an incident angle of 55.4°. When the grating is designed according the methodology described, it redirects to the transmitted light to the −1^(st) order mode with an angle of 34.6°, resulting in a 90° bend of the light.

Potential ranges for the incident angle and wavelength of a 90° coupler are shown in FIGS. 9 a and 9 b. In FIG. 9 a, line 912 corresponds to the s-polarization diffraction efficiency for the −1st order and line 914 corresponds to the s-polarization diffraction efficiency for the 0^(th) order with respect to the change in incident angle θ_(in). Line 916 corresponds to the p-polarization diffraction efficiency for the −1^(st) order and line 918 corresponds to the p-polarization diffraction efficiency for the 0^(th) order with respect to the change in incident angle θ_(in).

FIG. 9 a illustrates that the wavelength bandwidths of s-polarization are wider than those of p-polarization. This is due to the fact that the effective refractive index difference for s-polarization is larger than that for p-polarization in the grating. In FIG. 9 b, line 922 is the s-polarization diffraction efficiency for the −1^(st) order and line 924 is the s-polarization diffraction efficiency for the 0^(th) order with respect to the normalized wavelength variation A Line 926 is the p-polarization diffraction efficiency for the −1^(st) order and line 928 is the p-polarization diffraction efficiency for the 0^(th) order with respect to the normalized wavelength variation A. As such, the angular bandwidth for a transmittance larger than 50% is 9.40 for p-polarization and 18.60 for s-polarization. Wavelength bandwidth for a transmittance larger than 50% is 0.088λ for p-polarization and 0.212λ for s-polarization.

As a person skilled in the art will readily appreciate, the above description is meant as an illustration of the principles this application. This description is not intended to limit the scope or application of the invention in that the invention is susceptible to modification, variation and change, without departing from spirit of the invention, as defined in the following claims. 

1. A grating structure comprised of a base, the base having alternating ridges and grooves forming a grating, the grating having a ratio between the effective refractive index difference between s-polarization and p-polarization of about 1/3 thereby directing both s-polarization and p-polarization components of incident light to the −1^(st) order diffraction mode, where the incident light has an incident angle θ_(in) of between 40° and 90° and a wavelength λ=350-1600 nm.
 2. The grating structure according to claim 1, wherein the grating is in a Littrow mounting condition, such that a −1^(st) order diffraction mode has a first diffraction angle and a 0^(th) order diffraction mode has a second diffraction angle, wherein the first diffraction angle is equal to the second diffraction angle.
 3. The grating structure according to claim 1, wherein the −1st order diffraction mode has a diffraction angle that is about 90 degrees relative to the incident angle.
 4. The grating structure according to claim 1, wherein the base is a fused silica base and the ridges and grooves form a fused silica to air interface.
 5. The grating structure according to claim 4, wherein the incident angle θ_(in) is about 40° and the alternating ridges and grooves have a grating period p=0.75λ-0.81λ, a fill factor r=0.32-0.42, a grating height h=4.1λ-4.7λ.
 6. The grating structure according to claim 4, wherein the incident angle θ_(in) is about 50° and the alternating ridges and grooves have a grating period p=0.62λ-0.68λ, a fill factor r=0.49-0.59, a grating height h=4.1λ-4.7λ.
 7. The grating structure according to claim 4, wherein the incident angle θ_(in) is about 60° and the alternating ridges and grooves have a grating period p=0.55λ-0.61λ, a fill factor r=0.63-0.73, a grating height h=5.1λ-5.7λ.
 8. The grating structure according to claim 4, wherein the incident angle θ_(in) is about 70° and the alternating ridges and grooves have a grating period p=0.5λ-0.56λ, a fill factor r=0.34-0.44, a grating height h=2.8λ-3.4λ.
 9. A light assembly for a vehicle comprising: a non-polarized light source that generates rays of light; a grating having a base, the base having alternating ridges and grooves, the grating having a ratio between the effective refractive index difference between s-polarization and p-polarization of about 1/3 thereby directing both s-polarization and p-polarization components of the light to the −1^(st) order diffraction mode, where the light has an incident angle θ_(in) of between 40° and 70° and a wavelength λ=350-1600 nm.
 10. The light assembly according to claim 9, wherein the grating is in a Littrow mounting condition, such that a −1^(st) order diffraction mode has a first diffraction angle and a 0^(th) order diffraction mode has a second diffraction angle, wherein the first diffraction angle is equal to the second diffraction angle.
 11. The light assembly according to claim 9, wherein the −1^(st) order diffraction mode has a diffraction angle that is about 90 degrees relative to the incident angle.
 12. The light assembly according to claim 9, wherein the base is a fused silica base and the ridges and grooves form a fused silica to air interface.
 13. The light assembly according to claim 12, wherein the incident angle θ_(in) is about 40° and the alternating ridges and grooves have a grating period p=0.75λ-0.81λ, a fill factor r=0.32-0.42, a grating height h=4.1λ-4.7λ.
 14. The light assembly according to claim 12, wherein the incident angle θ_(in) is about 50° and the alternating ridges and grooves have a grating period p=0.62λ-0.68λ, a fill factor r=0.49-0.59, a grating height h=4.1λ-4.7λ.
 15. The light assembly according to claim 12, wherein the incident angle θ_(in) is about 60° and the alternating ridges and grooves have a grating period p=0.55λ-0.61λ, a fill factor r=0.63-0.73, a grating height h=5.1λ-5.7λ.
 16. The light assembly according to claim 12, wherein the incident angle θ_(in) is about 70° and the alternating ridges and grooves have a grating period p=0.5λ-0.56λ, a fill factor r=0.34-0.44, a grating height h=2.8λ-3.4λ.
 17. A method of producing a grating structure comprising: defining a wavelength; defining an incidence angle; determining a grating period based on a Littrow mounting condition; determining the relationship between the effective refractive index and the fill factor; identifying a fill factor where the ratio between the effective refractive index difference between s-polarization and p-polarization of about 1/3; determining the height of the grating; etching the grating based on the grating period, fill factor, and height.
 18. The method according to claim 17, further comprising calculating the relationship between the height of the grating and the diffraction efficiency and selecting the smallest height where the effective refractive index difference between s-polarization and p-polarization of about 1/3.
 19. The method according to claim 17, further comprising calculating the relationship between the aspect ratio of the grating and the diffraction efficiency and selecting the smallest aspect ratio where the effective refractive index difference between s-polarization and p-polarization of about 1/3.
 20. A method of coupling an unpolarized light beam comprising: providing a grating with a ratio of the effective refractive index difference between s-polarization and p-polarization of about 1/3; providing an unpolarized light beam at an incident angle between about 40° and 90°; diffracting the unpolarized light beam such that p-polarization components and s-polarization components of incident light are directed to the −1^(st) order diffraction mode.
 21. The method according to claim 20, wherein the grating is in a Littrow mounting condition.
 22. The method according to claim 18, wherein the −1^(st) order diffraction mode has a diffraction angle that is about 90 degrees relative to the incident angle. 